Activated carbon materials are commonly used to adsorb hydrocarbons and other impurities from gas streams (frequently air) and liquids. For these applications, the carbon is generally used in the form of granules. While activated carbon in the form of granules can perform the desired adsorption for many applications, there are some applications in which the granules have drawbacks. In some cases back pressure is a problem with the granules since the flow must follow a tortuous path. Some applications can result in considerable wear of the granules by attrition causing loss of material or bed packing. Furthermore, the fines which are generated as a result of attrition can block the flow path.
Another approach is to use an extruded activated carbon in the form of a cellular honeycomb. The honeycomb can readily be shaped by extruding fine powders of activated carbon with suitable binders. Such a shape allows for ease of flow of the gases through the honeycomb with little back pressure. Also, the geometry can be such as to allow easy access of the gases to all of the carbon for adsorption of the species to be removed. In the use of granules, the adsorbing species must diffuse into the center of the granule. This diffusion distance can be great compared to the thickness of the web of a honeycomb. Also, since a honeycomb is a solid piece, there should be little or no wear or attrition of the carbon.
Among the uses for such activated carbon honeycombs are the adsorption of hydrocarbon vapors in automotive applications. There are two automotive applications: (1) the evaporative emissions of vapors from the fuel system and the engine intake areas, and (2) cold start application.
In the case of evaporative emissions, the activated carbon adsorbs vapors given off from the fuel system while the vehicle is not operating, such as from the expansion and contraction of gases in the fuel tank with temperature swings. During refueling, the air which is displaced from the tank carries along considerable fuel as vapors which must be captured to meet future air pollution standards. The adsorbed species are then desorbed while the engine is operating and recycled back into the engine intake to be burned. Most vehicles today have an activated carbon canister filled with the granules to take care of some sources of vapors. However, this is not adequate to meet future requirements.
In the cold start application the activated carbon adsorbs hydrocarbons emitted during the initial 90 to 120 seconds after start-up of the engine. During this start-up period, the catalytic converter is not up to temperature for converting the hydrocarbons being emitted from the engine. Once the catalytic converter is up to temperature, the activated carbon can be removed from the exhaust system in a by-pass mode. The adsorbed hydrocarbons are desorbed from the activated carbon and are fed into the engine or into the exhaust ahead of the catalytic converter where they are converted to innocuous entities. The activated carbon is thus ready to adsorb hydrocarbons during the next cold start cycle.
In order to form an activated carbon honeycomb by extrusion, the carbon must be in the form of a fine powder. This can then be mixed with a liquid such as water and suitable plasticizers and binders. This plasticized mixture is then extruded through a die into the honeycomb shape, and dried.
Organic binders such as methylcellulose provide plasticity to the mixture. Such mixtures are soft and difficult to handle in the wet as extruded state before drying. Moreover, the bodies formed from such mixtures are relatively low in strength especially at elevated temperatures such as 250.degree. C. which are encountered in applications such as auto exhaust purification. This is a result of the degradation of the organic binders.
High levels of some binders, such as the organic binders described above, result in decreased surface area in the body. As a result, the adsorption efficiency of the activated carbon decreases. This is an important consideration in hydrocarbon adsorption applications.
It is highly desirable to improve the strength of the extruded honeycomb both in the extruded state for further processing and handling and also after drying to improve performance.
Clays have been used as binders in carbon mixtures to impart strength to the carbon body formed therefrom.
U.S. Pat. Nos. 4,259,299, 4,518,704, and Japanese patent application publication no. 57-122924 (1982) relate to activated carbon bodies in which clay binders are used.
U.S. Pat. No. 4,259,299 relates to a process for removing ozone using activated carbon product made from a mixture of activated carbon and zeolite and organic and/or inorganic binder. The organic binder can be lignin; the inorganic binder can be bentonite. The material is heated to 350.degree. C. to develop strength.
There remains a need to produce activated carbon bodies of improved strength to allow them to function effectively in high temperature applications such as in auto exhaust purification applications without sacrificing surface area and hence adsorption efficiency. The bodies must also have sufficient strength to be handled in the wet as-formed state to prevent deformation in handling. Also there remains a need to improve the handleability and formability characteristics of the forming mixtures.
The present invention meets these needs.